This invention relates to a composite article for use in a gas turbine engine, and more particularly to a method and apparatus providing a sealing arrangement suitable for use in the engine.
The efficient operation of a gas turbine engine depends upon a number of factors, not the least important of which is minimal leakage of gaseous fluid between various rotating and non-rotating elements of the engine. By way of example, leakage of air may occur between a rotating engine shaft and an adjacent portion of the stationary engine housing.
The use of labyrinth type seals to reduce leakage between rotating and non-rotating engine components is well-known in the art. Seal arrangements of this type typically utilize a series of sequentially arranged and closely spaced rotating knife-like edges. The knife-like edges are disposed closely adjacent to a facing material mounted on a stationary element of the engine and under engine operating conditions, the thermal expansion properties and the centrifugal forces acting on the sealing arrangement cause the knife-like edges of the rotating element to contact the stationary face element, penetrate the surface thereof and cut a groove therein. Under operating conditions, the knife-like edges rotate within the aforementioned grooves and hence leakage of air past the edges and the grooves is greatly inhibited and in most instances eliminated.
In seals of the type described, the facing material is of critical importance and must exhibit a number of acceptable physical characteristics. By way of example, the facing material must be resistant to erosion by particles entrained in the air or gas stream flowing through the engine at a high velocity. However, the facing material must not be abrasive to the extent that it wears away the knife-like edges forming part of the sealing arrangement. The facing material must have thermal expansion characteristics compatible with the stationary element to which it is bonded to insure against separation under high temperature conditions. In many instances, since modern day gas turbine engines utilize metallic elements having low coefficients of expansion, the facing element must have a low coefficient of expansion or a low modulus of elasticity. Finally, the facing material must be compatible with the high temperatures found in a typical seal location of a gas turbine engine. In some instances, these temperatures may range from 600.degree. to 1000.degree. Fahrenheit.
One particular facing material heretofore used in prior art sealing arrangements which has proven to be generally satisfactory is comprised of fibers of metal formed so as to exhibit a mat or felt-like construction. More specifically, this material is constructed by sintering a matrix of randomly oriented metallic fibers at a high temperature and at a reduced atmosphere to form a completely interlocked structure of metallic fibers having metallic bonds at all fiber contact points. The sintered material is further characterized by a bulk or apparent density substantially less than the density of the fibers themselves. The low density of sintered fiber metal materials is approximately in the range of 14% to 30% and hence these materials are differentiated from sintered powdered metals which have a density in excess of 30%.
Fiber metals of the type heretofore described have proved to be particularly well suited for use as face seals in aircraft gas turbine engines. Since such face seals include metal as a component, the seal exhibits the strength, rigidity and ruggedness necessary to withstand the rigors associated with a gas turbine engine. Furthermore, low apparent density of fiber metal seals is a characteristic which provides for compatibility between the face seal and the knife-edged tooth. More specifically, since the aforementioned fiber metal compact is comprised of a continuous metallic lattice disposed within a network of interconnected pores, the face seal is both slightly resilient and crushable and abradable. This construction permits rubbing contact between the face seal and a rotating knife-edged tooth to occur without excessive seal wear. Upon contact with a knife-edged tooth the pores of the face material adjacent to the contact point between the seal and the tooth collapse locally. This action reduces the contact force between the seal and the tooth. Additionally, since the face seal is only partially comprised of metallic elements, the remaining portion being a network of interconnected pores, the knife-edged tooth is only in minimal contact with the metallic elements. Since the metallic elements are primarily responsible for wear of the tooth, the face seal comprised of fibers of metal as hereinbefore set forth is well adapted to promote maximum life of the seal tooth.
While fiber metal face seals of the type described have generally proven to reduce leakage between zones in gas turbine engines they have not proven to be totally satisfactory. More specifically, while the interconnected network of pores in the material is responsible for the favorable wear patterns between the face seal and the tooth, the network permits the passage of air through the face seal material itself. Since even small amounts of air leakage may have a significant effect upon the performance of a modern day gas turbine engine, the interconnected porosity associated with sintered fiber metals face seals is a significant shortcoming. The present invention principally addresses this shortcoming without introducing alterations into the seal which would unfavorably affect the compatibility between the face seal and the rotating teeth.
Accordingly it is a principle object of the present invention to provide a new and useful material for use in a sealing arrangement in a gas turbine engine.
It is another object of the present invention to provide a face seal which is essentially impervious to the passage of air therethrough.